This invention relates to the recovery of hydrocarbons from an underground reservoir by cyclic injection and production processes, wherein formation permeability and the gas/oil ratio is lowered, and oil recovery increased with the use of one or more additives in the recovery fluid.
Numerous methods of enhanced oil recovery exist which involve the injection of a gaseous or a gaseous/liquid fluid into an underground formation. Recovery is often best where the fluid is injected at conditions so as to make the fluid miscible or conditionally miscible with the underground hydrocarbons. For non-thermal systems, the chief recovery fluid has been carbon dioxide.
Gaseous or gaseous/liquid recovery fluid methods may be divided into two types: drive processes and cyclic processes, which are also known as huff-n-puff or push/pull. In drive oil recovery processes, injection and production of fluids occur at different wells. In cyclic oil recovery processes, injection and production of fluids occur through the same well. Besides those structural differences, drive and cyclic processes are substantially different in that slugs of recovery fluid are designed differently, times of recovery are different, well patterns are different, costs are different, fluid velocities are different, and so forth.
Unlike drive recovery methods, cyclic processes are better suited for small oil reservoirs, particularly with the use of existing wells. The cost of recovery with a drive process in some smaller reservoirs, especially in deeper zones, may be so high as to make the reservoirs border-line candidates at best for oil recovery after primary or secondary recovery.
One of the earliest disclosures of a cyclic oil recovery process was in U.S. Pat. No. 3,480,081, wherein the flooding medium was water, brine or steam. The success of steam cyclic recovery processes inevitably lead to the cyclic injection and production of carbon dioxide with a soaking. U.S. Pat. No. 4,390,068 discloses such a carbon dioxide cyclic process. Cyclic carbon dioxide recovery has now become a commonplace event in the oil field.
Attempts to recovery heavy oils and hydrocarbons from tar sands have lead to a number of processes involving the injection of various solvents and hot fluids in "pressurization and drawdown" methods. These are similar to cyclic carbon dioxide methods in that various solvents and fluids are injected into the formation through a well to increase formation pressure. The fluids may or may not be allowed to soak in the formation prior to producing the injected fluids along with hydrocarbons through the same well. U.S. Pat. No. 4,324,291 is one example of these processes.
U.S. Pat. No. 4,913,235 teaches a method for increasing the viscosity of an injected solvent by the addition of a polydimethylsiloxane type polymer and a cosolvent such as toluene. The drive process disclosed therein is somewhat effective in countering horizontal conformance problems resulting from viscosity differences between injected solvent and reservoir fluids, but is relatively ineffective in minimizing preferential movement of injected solvent through high permeability regions.
A variation of the above method is disclosed in U.S. Pat. No. 5,095,984, wherein the enhanced oil recovery drive process is designed so that the viscosifying surfactant or polymer, such as polydimethylsiloxane, is soluble in the drive solvent under the injection conditions of temperature and pressure, but is substantially insoluble under the formation conditions of temperature and pressure. U.S. Pat. No. 5,095,984 discloses that polydimethylsiloxane or other solute can be precipitated in a drive process by (1) a temperature increase to reservoir temperature, or (2) dilution or separation of the cosolvent, but has no satisfactory solution for precipitation by varying pressure. The possibility of cyclic injection and production obviously never occurred to the author of this reference, for after he states that the lowering of pressure may achieve some precipitation, he concludes, "Unfortunately, any substantial pressure reduction in an underground formation is not a simple or quick task." He suggests the possibility of a sudden pressure pulse to drop solute in the desired location. See Col. 8:56-66.